Engine Speed Sensor, Checking

Observe all safety precautions: SAFETY PRECAUTIONS 

View clean working conditions: CLEAN WORKING CONDITIONS 

Prior to repair work, perform a preliminary check to verify the condition. Refer to PRELIMINARY CHECK .

Special tools and workshop equipment required 

• multimeter.
• Wiring diagram.

Test requirements 

• The Motronic Engine Control Module (ECM) - J623- fuses OK.
• Battery voltage at least 12.5 volts.
• All electrical consumers such as, lights and rear window defroster, switched off.
• Vehicles with automatic transmission, shift selector lever into position "P" or "N".
• A/C switched off.
• Ground connections between engine/transmission/chassis OK.
• Ignition switched off.

Function 

The Engine Speed (RPM) Sensor -G28- detects RPM and reference marks. Without an engine speed signal, the engine will not start. If the engine speed signal fails while the engine is running, the engine will stall.

Test procedure 

-- Perform a preliminary check to verify the customers complaint. Refer to PRELIMINARY CHECK 

Start diagnosis 

-- Connect the scan tool.

-- Start the engine and allow it to idle.

-- Using the scan tool, check the engine speed:

Diagnostic text		Specified value
Engine revolutions per minute (RPM)		Idle speed

-- Switch ignition off.

If the specified value was Not obtained:

Checking internal resistance 

-- Disconnect the Engine Speed (RPM) Sensor -G28- electrical harness connector.

-- Using a multimeter, check the Engine Speed (RPM) Sensor -G28- terminals 1 to 3 for resistance.

Specified value: 1540 Ω +/- 300 ohms

If the specified value was Not obtained:

-- Replace the Engine Speed (RPM) Sensor -G28-. Refer to the Repair information.

If the specified value was obtained:

Checking wiring 

If the manufacturers test box is being used, perform the following step.

-- Install the test box.

If the manufacturers test box is not being used, perform the following step.

-- Remove the Motronic Engine Control Module (ECM) -J623-. Refer to the Repair information.

-- Using a multimeter, check the Engine Speed (RPM) Sensor -G28- electrical harness connector terminals to the Motronic Engine Control Module (ECM) -J623- electrical harness connector T60 terminals for an open circuit.

Engine Speed (RPM) Sensor -G28- electrical harness connector terminals	Motronic Engine Control Module (ECM) -J623- electrical connector T60 terminals or test box socket
1	25
2	52
3	53

Specified value: 1.5 Ω Max.

If the specified value was Not obtained:

-- Check the wiring for an open circuit, high resistance or a short circuit to Battery or Ground.

-- Check the electrical harness connector for damage, corrosion, loose or broken terminals.

-- If necessary, repair the faulty wiring connection.

If no malfunction is detected in the wiring:

-- Remove the Engine Speed (RPM) Sensor -G28- and check the sensor wheel for proper seating, damage and run out. Refer to the Repair information.

If the sensor wheel is OK:

-- Replace the Motronic Engine Control Module (ECM) -J623-. Refer to the Repair information.

Final procedures 

After the repair work, the following work steps must be performed in the following sequence:

1. Check the DTC memory. Refer to DIAGNOSTIC MODE 03 - READ DTC MEMORY .
2. If necessary, erase the DTC memory. Refer to DIAGNOSTIC MODE 04 - ERASE DTC MEMORY .
3. If the DTC memory was erased, generate readiness code. Refer to READINESS CODE .